Today there is an increasing trend of data communication between communicating units in different data communications systems. Typical examples are data communication between computers connected over a communications system such as Internet or a local area network and data communication between mobile user equipment and a communications server or node or another mobile user equipment over a radio communications system.
Many application protocols used in the data communication require that certain information and data is included in the data messages transmitted over the communications system. This data could be required for enabling efficient delivery of the data messages throughout the system and/or could be used by the receiving communications unit for interpreting and processing the received data messages. As a consequence many of the communicated data messages include fields that always comprise more or less the same data for a given pair or combination of communications units. This is generally no problem in communications systems with bandwidth rich communications links. However, in radio communications systems and other systems with typically a limited amount of available communications resources there is a general desire to reduce the amount of data that has to be transmitted between communicating units.
In addition, multimedia data messages are typically engineered for bandwidth rich links. Today, multimedia services employing such multimedia messages are also emerging for radio communications systems with, generally, poor bandwidth links. As a result, problems may arise since the multimedia messages have not been optimized in terms of size for such radio communications systems. For example, typical Session Initiated Protocol (SIP) messages range from a few hundred bytes up to two thousand bytes or more. With the use of these protocols in wireless mobile units as part of radio communications networks, the large message size is problematic. With low-rate Internet Protocol (IP) connectivity the transmission delays are significant. Taking into account retransmissions, and the multiplicity of messages that are required in some flows, call setup and feature invocation are adversely affected.
A possible solution could then be to employ a compression algorithm for compression of transmitted (multimedia) data messages and, thus, a reduction of the amount of data that has to be transmitted for each such message.
There are several different such compression techniques available for data compression in e.g. mobile communications systems. A typical example is the signal compression SigComp that was recently developed and is further described in the Request for Comments (RFC) documents [1, 2]. SigComp is a compression solution for compressing data messages generated by application protocols such as SIP, Session Description Protocol (SDP) and the Real Time Streaming Protocol (RTSP) and offers a robust, lossless compression of such application messages.
However, even the data messages compressed by SigComp and other compression protocols still comprise certain information fields that typically always comprise the same data for a given communications units pair. Although a compression technique reduces an overall size of a data message to be communicated over a communications system there may still be problems with the limited availability of communications resources, in particular for radio communications systems and other bandwidth poor systems. Thus, there is a need to further reduce the size of communicated data messages and compressed data messages.